Novel Cyanide-Free Electroplating Process for Zinc and Zinc Alloy Die-Cast Components
The proposed invention comprises a process for plating upon zinc die cast articles without the use of cyanide in the plating solution The process proposes the plating of the zinc die cast articles with a zinc alloy layer first, followed by plating with copper, nickel, chromium, tin or brass. The preferred zinc alloy initial coating is zinc-nickel.
The present invention is directed to a method for covering an article manufactured from zinc or zinc alloy with an adherent metallic coating, said method being substantially free from cyanide ions. The coating of the invention renders the article suitable for subsequent electroplating in other metals such as copper, nickel and chromium.
BACKGROUND OF THE INVENTIONZinc and zinc alloy products are generally produced as die castings and are commonly used in applications such as automotive parts or fittings. To enhance the decorative appearance of such articles and to provide them with protection against corrosion, said articles are commonly coated with other metals by the process of electrolytic or electroless plating. The most common metals for this application are copper, nickel, chromium and brass. Further enhancements in appearance or corrosion protection may be obtained by the use of organic coatings.
Zinc and zinc alloy articles are traditionally electroplated with a first coating of copper, which is applied from a process that contains cyanide ions. The copper solution is alkaline in order to minimize chemical attack on the substrate and the cyanide ions are necessary to provide a stable complex of copper (I) ions. Efforts have been made to replace copper cyanide electroplating solutions in order to eliminate the use of cyanide ions due to their high toxicity.
Suitable stable aqueous complexes of copper (I) ions to replace cyanide ions in alkaline copper electroplating baths have proved elusive. U.S. Pat. No. 5,750,018 describes an alkaline copper (l) solution using imides such as succinimide or a hydantoin such as dimethylhydantoin, but these solutions require the permanent presence of a reducing agent such as an alkali sulphite or bisulphite, or an amine such as hydroxylamine or hydrazine. However the copper (I) ion remains inherently unstable, oxidation to copper (II) occurs readily, and such baths are not robust, requiring close monitoring and maintenance. They have not found favour in industrial applications.
Alkaline copper (II) baths containing complexants have been suggested, with pyrophosphate being a well know example. However the pyrophosphate complex is not stable enough to prevent the chemical deposition of copper from copper(II) onto the zinc-based substrate, with the corresponding chemical dissolution of zinc from the surface, even with a cathodic potential applied prior to immersion in the bath. Such immersion deposits produce poorly adherent coatings that either flake off or exhibit blistering.
U.S. Pat. No. 3,928,147 claims a method of immersing the zinc articles in a solution of an organophosphorus chelating agent prior to immersion in the cyanide-free copper(II) electroplating bath in order to inhibit the immersion deposition of copper.
More recently U.S. Pat. No. 4,469,569 and U.S. Pat. No. 4,521,282 have described the use of organophosphonate complexing agents in a copper plating bath, and U.S. Pat. No. 6,054,037 describes an organophosphonate bath modified by the addition of halogen ions to stabilise copper(I) ions in the cathode film. These types of bath are found to be suitable to electroplate onto steel and in some instances are found to work on zinc substrates with a cathodic potential applied prior to immersion in the bath. However, in general, immersion deposits still occur in the aforementioned baths even with the application of cathodic potential prior to immersion, and in this respect the foregoing types of copper(II) baths have not found general use for the application of electroplating on zinc die-cast articles.
Most recently U.S. Patent Application 2008/0156652 describes an alkaline electroplating bath of copper (II) salts with a tartrate complexant and an additive selected from phosphate or hypophosphite, wherein the article is electroplated at 0.002 to 0.02 A/dm2. However the inventors of the present invention have found this system to also give immersion deposits with inherently poorer adhesion than electroplated deposits.
Alternative approaches to replacing cyanide have been directed to replacing copper with a different metal coating. GB2272001 describes an electrolyte containing zinc and tin for use on zinc die-casts prior to electroplating with alkaline cyanide-free copper, which would be believed to be of the copper(II) pyrophosphate type.
U.S. Pat. No. 6,827,834 proposes the use of an alkaline pyrophosphate nickel bath to deposit an immersion nickel coating prior to a pyrophosphate copper electroplating stage. However, immersion deposits are inherently less adherent than electroplated deposits and in addition, any deposit of a different metal needs to be thick enough to provide a non-porous film in order to prevent further immersion deposition of copper from the subsequent copper(II) solution. Immersion deposits are not thick enough to give non-porous layers. Thus this method still suffers from poor adhesion and a general lack of robustness when compared to the current cyanide-based electroplating technique.
Accordingly, U.S. Patent Application 2006/0096868 describes a mildly acidic near-neutral nickel electroplating bath used to deposit a thick nickel layer on the zinc die-cast article. Although the deposits from this type of bath are thick enough to protect the substrate from attack in subsequent process stages, the electrolyte has poor throw into recesses and therefore some zinc substrate can remain exposed on complex-shaped parts. Additionally the deposits from this electrolyte exhibit very high internal stress and poor ductility.
Although the above prior art describes the production of adherent deposits, a typical electroplater will be required to test the adhesion of the coatings on zinc die-cast articles by heating the articles, for example for 1 hour. After the heating stage the articles are immediately plunged into water at ambient temperature. A common heating temperature for such a test is 180° C. This heat and quench test can reveal a lack of adhesion of all the coatings of the prior art, demonstrated by the appearance of large blisters on the articles. The current method of using cyanide-containing copper(I) baths for electroplating the first layer onto the article provides blister-free results from this test.
Thus there remains a requirement to provide a process for electroplating zinc die-cast articles without the use of baths containing copper cyanide, and which is capable of demonstrating excellent adhesion when subjected to the aforementioned heat and quench test. Such a process must also be robust enough to withstand continuous use under typical industrial production conditions with a minimum of technical control and maintenance.
The present invention is directed to a method for producing an adherent metallic coating on a zinc or zinc alloy article without the use of cyanide ions in the process. The zinc or zinc alloy article is first cleaned in the normal manner and then electroplated with a zinc alloy coating, said zinc alloy coating being preferably a zinc-nickel alloy. The said zinc alloy coating has good adhesion to the substrate and provides a metallic layer suitable for subsequent electroplating stages.
SUMMARY OF THE INVENTIONThe present invention describes an electroplating bath and method which provide for producing a zinc alloy coating on a zinc die-cast substrate, said coating providing a suitable base for subsequent electroplating, without the use of cyanide ions. Preferably the zinc alloy coating is a zinc-nickel alloy. The method comprises the following steps;
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- (a) cleaning and activation of the zinc die-cast article;
- (b) electroplating of the article in a bath comprising zinc ions, alloying metal ions (which are preferably nickel ions), and counter ions;
- (c) subsequently coating the article by electrolytic or electroless means with one or more of the following; copper, nickel, chromium, tin, brass, or other metal as desired and optionally a final organic coating to achieve an attractive and corrosion resistant finish on the article.
This invention describes a method of treating zinc die-cast articles to produce an adherent coating suitable for the application of subsequent electroplated metallic coatings without the use of solutions containing cyanide ions. The method of the invention generally includes the steps of:
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- (a) optionally, but preferably, cleaning and activating the zinc die-cast article;
- (b) electroplating the article in a bath comprising zinc ions, alloying metal ions (which are preferably nickel ions), and counter ions;
- (c) subsequently coating the article by electrolytic or electroless means with further metallic layers and optionally a final organic coating.
The cleaning and activating step is preferred to provide a surface that is suitable for electroplating. Defects such as lack of adhesion, porosity, roughness, dark spots and non-uniform coating are likely to occur on poorly prepared parts. The surface preparation process also serves to activate the surface of the part so that it is optimally receptive to the deposition of the metal coating.
The zinc die-cast articles are first cleaned in a neutral or alkaline degreasing cleaner to remove oils from the surface of the articles and also any residual buffing compound that may be present from post-casting buffing operations. The articles are thoroughly rinsed and then activated to remove surface oxides by a short immersion in a weakly acid solution such as 5-10% sulphuric acid. Thorough rinsing is preferred between all cleaning stages and also prior to electroplating to remove all traces of acid and alkali from any porous areas on the article. After cleaning and activating, the zinc die-cast article is electroplated by making it the cathode in an aqueous solution containing zinc ions, alloying metal ions, which are preferably nickel ions, and counter ions.
The source of zinc and alloying ions in the invention are any water soluble zinc or alloying metal salts. Of particular preference for zinc are zinc chloride, zinc sulphate, zinc acetate and alkali metal zincates. The preferred source of the alloying ions is the chloride, sulphate or acetate salt of the chosen alloying metal, preferably nickel. The concentration of zinc ions in the electrolyte is generally between 2 and 100 g/l, and the concentration of alloying metal is generally between 0.2 and 100 g/l. The counter ions merely refers to the anions (eg. chloride, sulfate, acetate, etc.) that are associated with the metal ions in the salt chosen. Other useful alloying metals include cobalt, copper and iron.
In addition to the zinc and alloying metal salts the solution may contain other salts and additives, for example (i) sources of hydrogen ion or hydroxide ion to adjust the pH of the electrolyte, (ii) buffering compounds such as ammonium ions, borate ions or organic acid species, (iii) complexing agents such as an amine to prevent precipitation of metal hydroxides, (iv) additional inorganic salts to improve the conductivity of the electrolyte, and (v) wetting agents and brightening agents.
In a first embodiment of the invention, the electrolyte is composed of a water soluble zinc salt providing zinc ions preferably in the concentration range of 10-100 g/l, a water soluble alloying metal salt providing alloying metal ions preferably in the concentration range of 10-100 g/l, a buffering compound and optionally further inorganic salts, wetting agents and brighteners. Preferably the zinc salt is zinc chloride, the alloying metal salt is most preferably nickel chloride or nickel sulphate, and ammonium chloride or boric acid are the preferred buffering agent. The electrolyte is operated at a mildly acidic pH in the range 4.5-7.0, preferably about 5.0-5.5, and can be operated in the temperature range of 10-90° C. but preferably at 20-30° C.
In a second embodiment of the invention, the electrolyte is composed of an alkali metal zincate providing zinc ions preferably in the concentration range of 2-30 g/l, a water soluble alloying metal salt providing alloying metal ions preferably in the concentration range of 0.2-5 g/l, an amine compound to act as a chelating agent for the alloying metal ions, an alkali metal hydroxide, optionally an alkali metal carbonate and optionally wetting and brightening agents. Preferably the alkali metal zincate is sodium or potassium zincate, the alloy metal salt is nickel sulphate, the chelating agent is a polymeric amine or a substituted ethylenediamine compound and the alkali metal hydroxide is sodium or potassium hydroxide. The electroplating solution is operated at a pH of between 10 and 14 and can be operated in the temperature range of 10-90° C. but preferably at 20-30° C.
Both embodiments of the invention provide a zinc alloy deposit which normally consists of 70-90% zinc and 10-30% of alloying metal, preferably nickel.
The anode in the electroplating baths may be either metallic zinc, the alloying metal which is preferably nickel, or insoluble anodes for example titanium coated with mixed metal oxides. For the first embodiment of the invention a zinc anode is preferred, and for the second embodiment of the invention an anode of the alloying metal, which is most preferably nickel, or a coated titanium anode is preferred.
The articles are electroplated in the solutions of the invention at current densities of between 0.1 and 5.0 A/dm2, preferably between 1 and 4 A/dm2, and the plating time is normally 2-10 minutes, preferably 4-8 minutes. Under these conditions an adherent zinc alloy, preferably a zinc-nickel alloy, is deposited. Typically the deposit contains 10-30% of alloying metal with the remainder being zinc. Both embodiments of the invention have minimal or no attack on the zinc die-cast articles and provide for deposits with excellent coverage into the recesses of complex-shaped parts, thereby providing optimum protection during subsequent processing stages.
Although both embodiments of the invention can produce the desired coating, the preferred embodiment is the mildly acidic bath described in the first embodiment.
After electroplating in the bath of the invention the articles are subsequently electroplated with any suitable electroplating bath, however due to the coating of the invention being 70-90% zinc, processing in alkaline baths is preferred with pyrophosphate copper being most preferred. For optimum adhesion of the pyrophosphate copper deposit to the coating of the invention, it is preferred to apply a voltage to the article before immersion in the copper pyrophosphate bath.
The following non-limiting examples demonstrate the application of the invention.
EXAMPLESIn the following examples, unless specified differently, all zinc die-cast parts have been treated according to the following procedure;
Neutral soak cleaner to degrease (65° C., 5 minutes)
Alkaline electrolytic (anodic) cleaner (2 volts, 30 seconds)
Rinse
Activator (25 g/l sodium bisulphate and 2 g/l sodium fluoride, room temperature, 30 seconds)
Rinse
Electroplate as described in the example
Rinse
Electroplate in alkaline copper (II) pyrophosphate bath (3 A/dm2, 20 minutes)
Electroplate in conventional Watts bright nickel bath (4 A/dm2, 13 minutes)
Electroplate in chromium (conventional hexavalent chromium bath, 10 A/dm2, 5 mins)
Rinse and dry the part.
Heat the part to 180° C. for 1 hour
Dip the part in cold water
Assess the deposit for blistering and other indicators of poor adhesion
Examples of the Prior Art Example 1A zinc die-cast part was pretreated and then electroplated in a conventional alkaline cyanide copper(I) electroplating bath at 1 A/dm2 for 5 minutes at a temperature of 55° C. This example is representative of the conventional established process of the prior art.
Example 2An alkaline copper(II) pyrophosphate electrolyte was prepared as follows;
A zinc die-cast part was pretreated and then electroplated in the solution at 3.5 A/dm2 for 20 minutes at a temperature of 55CC followed by conventional nickel and chromium plating.
Example 3An electrolyte was prepared as follows;
A zinc die-cast part was pretreated and then electroplated in the solution for 15 minutes at 0.5 A/dm2 and a temperature of 55° C. A cathodic potential was applied to the part prior to immersion. This electroplating stage was followed by conventional nickel and chromium plating. This example is believed to be representative of the prior art of U.S. Pat. No. 4,469,569 and U.S. Pat. No. 4,521,282.
Example 4An electrolyte was prepared as follows;
A zinc die-cast part was pretreated and electroplated in the solution for 15 minutes at 0.5 A/dm2 and a temperature of 55° C. A cathodic potential was applied to the part prior to immersion. This electroplating stage was followed by conventional nickel and chromium plating. This example is believed to be representative of the prior art of U.S. Pat. No. 6,054,037.
Example 5A solution was prepared as follows;
A zinc die-cast part was pretreated and then immersed in the above solution for 1 minute prior to being electroplated in the solution of Example 3 for 15 minutes at 0.5 A/dm2 and a temperature of 55° C., followed by conventional nickel and chromium plating. This example is believed to be representative of the prior art of U.S. Pat. No. 3,928,147.
Example 6An electrolyte was prepared as follows;
A zinc die-cast part was pretreated and immersed in the solution. A displacement copper deposit formed immediately upon immersion into the solution. The part was electroplated in the solution for 10 minutes at 0.01 A/dm2 and a temperature of 30° C. and subsequently electroplated by conventional pyrophosphate copper, nickel and chromium plating. This example is believed to be representative of the prior art of U.S. Patent Application 2008/0156652.
Example 7An electrolyte was prepared as follows;
A zinc die-cast part was pretreated and electroplated in the solution for 2 minutes at 2.0 A/dm2 and a temperature of 55° C. This electroplating stage was followed by conventional pyrophosphate copper, nickel and chromium plating. This example is believed to be representative of the prior art of GB2272001.
Example 8A solution was prepared as follows;
A zinc die-cast part was pretreated and immersed in the solution for 3 at a temperature of 50° C. An immersion nickel deposit formed. This stage was followed by conventional pyrophosphate copper, nickel and chromium plating. This example is believed to be representative of the prior art of U.S. Pat. No. 6,827,834.
Example 9A zinc die-cast part was pretreated and electroplated in the solution of example 8 for 10 minutes at 1.0 A/dm2 and a temperature of 50° C. This electroplating stage was followed by conventional pyrophosphate copper, nickel and chromium plating.
Example 10A solution was prepared as follows;
A zinc die-cast part was pretreated and electroplated in the solution for 5 minutes at 4.0 A/dm2 and a temperature of 55° C. and subsequently electroplated in conventional pyrophosphate copper, nickel and chromium plating. This example is believed to be representative of the prior art of U.S. Patent Appl. 2006/0096868.
An electrolyte was prepared as follows;
A zinc die-cast part was pretreated and electroplated in the solution for 10 minutes at 2.0 A/dm2 and a temperature of 30° C. A cathodic potential was applied to the part prior to immersion. This electroplating stage was followed by conventional pyrophosphate copper (with a cathodic potential applied prior to immersion), nickel and chromium plating.
Example 12An electrolyte was prepared as follows;
A zinc die-cast part was pretreated and electroplated in the solution for 10 minutes at 1.0 A/dm2 and a temperature of 30° C. A cathodic potential was applied to the part prior to immersion. This electroplating stage was followed by conventional pyrophosphate copper (with a cathodic potential applied prior to immersion), nickel and chromium plating.
Example 13An electrolyte was prepared as follows;
A zinc die-cast part was pretreated and electroplated in the solution for 5 minutes at 2.0 A/dm2 and a temperature of 20° C. A cathodic potential was applied to the part prior to immersion. This electroplating stage was followed by conventional pyrophosphate copper (with a cathodic potential applied prior to immersion), nickel and chromium plating.
Example 14An electrolyte was prepared as follows;
A zinc die-cast part was pretreated and electroplated in the solution for 5 minutes at 4.0 A/dm2 and a temperature of 25° C. A cathodic potential was applied to the part prior to immersion. This electroplating stage was followed by conventional pyrophosphate copper (with a cathodic potential applied prior to immersion), nickel and chromium plating.
Example 15An electrolyte was prepared as follows;
A zinc die-cast part was pretreated and electroplated in the solution for 10 minutes at 1.0 A/dm2 and a temperature of 25° C. A cathodic potential was applied to the part prior to immersion. This electroplating stage was followed by conventional pyrophosphate copper (with a cathodic potential applied prior to immersion), nickel and chromium plating.
Example 16An electrolyte was prepared as follows;
A zinc die-cast part was pretreated and electroplated in the solution for 5 minutes at 2.0 A/dm2 and a temperature of 25° C. A cathodic potential was applied to the part prior to immersion. This electroplating stage was followed by conventional pyrophosphate copper (with a cathodic potential applied prior to immersion), nickel and chromium plating.
The foregoing description and examples are provided by way of illustration only. Although the invention has been described with reference to particular and preferred features and embodiments it will be understood to those skilled in the art that these are not intended as limitations of the scope of the invention.
Claims
1. A process for plating upon a zinc die-cast article, said process comprising:
- (a) cleaning said article;
- (b) electroplating said article in an aqueous solution comprising (i) zinc ions, (ii) nickel ions, and (iii) counter ions, by making said article a negative cathode in said solution to produce a zinc-nickel alloy coating thereon; and
- (c) electroplating said article with a metal selected from the group consisting of copper, nickel, chromium, tin, and alloys of the foregoing.
2. (canceled)
3. (canceled)
4. A process according to claim 1 wherein the article is made cathodic prior to contacting it with the aqueous solution of step (b).
5. A process according to claim 18 wherein the article is made cathodic prior to contacting it with the aqueous solution of step (b).
6. A process according to claim 1, wherein the source of the nickel ions in step (b) is selected from the group consisting of nickel chloride, nickel sulfate and nickel acetate.
7. A process according to claim 6, wherein the nickel ions are present in the aqueous solution of step (b) at a concentration of between about 10 g/l and about 100 g/l.
8. A process according to claim 7, wherein the nickel ions are present in the aqueous solution of step (b) at a concentration of between about 50 g/l and about 55 g/l in the aqueous solution in step (b).
9. A process according to claim 1 wherein the source of the zinc ions in step (b) is a water soluble zinc salt.
10. A process according to claim 9 wherein the water soluble zinc salt is selected from the group consisting of zinc chloride, zinc sulfate, zinc acetate and alkali metal zincates.
11. A process according to claim 9 wherein the aqueous solution of step (b) comprises a buffering compound selected from the group consisting of ammonium ions and borate ions.
12. A process according to claim 9 wherein the water soluble zinc salt is present in the aqueous solution of step (b) at a concentration of between about 10 g/l and about 100 g/l.
13. A process according to claim 12 wherein the water soluble zinc salt is present in the aqueous solution of step (b) at a concentration of between about 45 g/l and about 50 g/l.
14. A process according to claim 1 wherein the source of the zinc ions of step (b) is an alkali metal zinc ate; and
- wherein the aqueous solution of step (b) additionally comprises an amine compound.
15. A process according to claim 14, wherein the alkali metal zincate is selected from the group consisting of sodium zincate and potassium zincate.
16. A process according to claim 14, wherein the alkali metal zincate is present in the aqueous solution of step (b) at a concentration of between about 2 and about 30 g/l.
17. A process according to claim 16, wherein the alkali metal zincate is present in the aqueous solution of step (b) at a concentration of between about 8 and about 12 g/l.
18. A process for plating upon a zinc die-cast article, said process comprising:
- (a) cleaning said article;
- (b) electroplating said article in an aqueous alkaline solution comprising (i) zinc ions, (ii) alloy metal ions, and (iii) counter ions, by making said article a negative cathode in said solution to produce a zinc-nickel alloy coating thereon; and
- (c) electroplating said article with a metal selected from the group consisting of copper, nickel, chromium, tin, and alloys of the foregoing, wherein said aqueous alkaline solution has a pH in the range of about 4.5 to about 7.0.
19. A process according to claim 18, wherein the pH of the aqueous solution in step (b) is between about 5.0 to about 5.5.
20. A process according to claim 18 wherein the concentration of zinc ions in the aqueous solution of step (b) is from about 10 to about 100 g/l.
21. A process according to claim 1 wherein the concentration of alloy ions in the aqueous solution of step (b) is from about 10 to about 100 g/l.
Type: Application
Filed: Oct 2, 2008
Publication Date: Apr 8, 2010
Inventors: Anthony J. Rowan (Leicestershire), Roderick D. Herdman (Staffordshire), Craig Robinson (Coventry)
Application Number: 12/244,156
International Classification: C25D 3/56 (20060101);
